rebase -i: teach --autosquash to work with amend!

rebase -i: teach --autosquash to work with amend!

If the commit subject starts with "amend!" then rearrange it like a
"fixup!" commit and replace `pick` command with `fixup -C` command,
which is used to fixup up the content if any and replaces the original
commit message with amend! commit's message.

Original-patch-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Mentored-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Charvi Mendiratta <charvi077@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>